The present invention relates to optical communications networks and more particularly to a device for extracting, from a stream of optical signals comprising a plurality of carriers, one or more carriers, modulated by corresponding information signals, and inserting into the stream one or more other carriers, modulated in turn by corresponding information signals which are normally different from the preceding ones.
These devices are commonly known in the art by the abbreviation OADM, from the initials of xe2x80x9cOptical Add-Drop Multiplexerxe2x80x9d. The devices form a key element for making nodes with wavelength-based routing functions in wavelength division multiplexing communications networks: in this case they are used for extracting from the multiplexed stream one or more channels containing the information addressed to a user or users connected to the node, while allowing the remaining channels to pass unaltered, and adding to the multiplexed flow one or more channels which carry the information generated within the node. In practice, to optimize the use of the optical band, it is convenient to allocate the extracted channel and the added channel (or each pair consisting of an extracted channel and an added channel) to the same wavelength position.
Among the various characteristics which a device of this type must have, the scalability and modularity in respect of extracted and inserted channels are particularly important for the purposes of permitting a gradual and economical increase of the capacity of the node in accordance with the growth in traffic generated and received in the node. The term xe2x80x9cscalabilityxe2x80x9d denotes the capacity to increase the number of extracted and inserted channels without degrading the channels already present (including both those in transit and the extracted and inserted channels). The term xe2x80x9cmodularityxe2x80x9d denotes the possibility of increasing the capacity simply by adding replicas of a basic module for a single channel.
A typical system for the construction of these basic modules makes use of an element sensitive to the wavelength of the channel to be extracted and inserted, connected between a pair of circulators, one of which receives the multiplexed stream. In one example, described in European patent EP-B 0 638 837, the wavelength-sensitive element is a tunable band-pass filter of the Fabry-Perot resonant cavity type formed in the fibre, which transmits the resonance wavelength and reflects all the others. With this arrangement, the channels in transit pass out, by reflection in the filter, through a port of the circulator through which the multiplexed stream enters the device, while the channel to be extracted is transmitted to the second circulator, from which it passes out. In a symmetrical way, the new channel is introduced through the second circulator and is transmitted by the filter, thus being inserted into the stream in transit.
In more recent versions of the same system, the wavelength-sensitive element is an in-fibre Bragg grating, which reflects the resonance wavelength and transmits all the others, so that the channel to be extracted is reflected by the grating towards the same circulator which has received the multiplexed stream and passes out of it, while the channels in transit proceed towards the second circulator, through which is inserted the new channel which, by reflection in the grating, will pass out of the second circulator together with the channels in transit.
A structure of this type is described, for example, in the paper xe2x80x9cAdd-Drop Ottici per reti WDMxe2x80x9d, presented by G. Bendelli et al. at the xe2x80x9cFotonica 97xe2x80x9d conference in Rome, Italy, 20-23 May 1997, and published on pp. 18 ff. of the conference proceedings.
Because of the excellent spectral characteristics of in-fibre Bragg gratings and the high isolation between the ports of the circulators, it is possible to construct devices having high performance in terms of both wavelength selectivity and inter-channel crosstalk.
One limitation present in the aforementioned devices is the low scalability in terms of the increase of the channels extracted and inserted in a node, since this would require either the insertion of additional elements in series between the circulators or the insertion of replicas of the device in series.
The first solution, although particularly simple, has two main drawbacks:
the interruption, throughout the time required by the operations of adding the elements, of the connections supported by the channels passing through the node (both those in transit and those inserted or extracted, depending on whether the grating is added upstream or downstream of the existing grating);
the necessity of separating the channels extracted from the single extraction port.
The second solution, which is described in the European patent cited above, also requires the interruption of the traffic during the operations of adding elements; it also entails a redundancy of expensive optical components such as optical circulators.
The object of the present invention is a new configuration of a wavelength extraction-insertion device based on Bragg gratings and circulators, intended to improve the properties of both scalability and modularity of the design in terms of the number of channels extracted and inserted, while reducing to a minimum the periods of interruption of the service and not requiring the replication of expensive components.
The device according to the invention comprises:
wavelength-sensitive means for selecting the carrier or carriers within the stream
means for receiving the stream and extracting the carrier or carriers, which receive the multiplexed stream and transfer it to the selection means, and send the selected carrier or carriers to user devices; and
output means for the multiplexed stream after the extraction and insertion of the carrier or carriers,
and is characterized in that the said means for receiving the stream and extracting a carrier and the said selection means are inserted in a structural module which is replicated for each of the carriers to be inserted and extracted and in which the selection means are capable of selecting a corresponding carrier which is sent to the exterior by the receiving and extracting means of the module, and in that each module also comprises connection points, located on the path of the multiplexed stream, which are accessible from the exterior and are designed for the connection of a plurality of modules in series, in such a way as to establish for the multiplexed stream a first path which is a direct path between an input of the module and an input of the receiving and extracting means of the module to which it belongs, if this module is the only one or is the last of the series of modules, or, when the said series of modules is present, and for all the modules except the last, a second path which passes through the receiving and extracting means and the selection means of the subsequent modules.